Cell Membrane Structure and Function

Cell Walls and Cell Membranes

The cell membrane is typically the outermost barrier of the cell, but some cells also have a cell wall for additional support and protection.

• Cell wall is found in:

  • Plants – composed of cellulose

  • Fungi – composed of chitin

  • Prokaryotes – composed of a variety of macromolecules

• The cell wall mainly provides structural support and protection, not a barrier to molecules.

The Plasma Membrane

The plasma membrane surrounds the cell and regulates interaction with the environment.

• Functions of the plasma membrane:

  1. Isolates the cell environment

  2. Regulates exchange between inside and outside

  3. Communicates with other cells

  4. Identifies the cell type

Structure of the Plasma Membrane

• Composed mainly of lipids and proteins.

• Phospholipids are the primary lipid, forming phospholipid bilayers that create a semi-permeable barrier.

• Biological membranes also contain proteins, which serve various functions.

Fluid Mosaic Model

The Fluid Mosaic Model (proposed by Singer and Nicolson) describes the structure of cell membranes:

• Proteins are embedded in a fluid lipid bilayer.

• Hydrophilic zones: polar head groups + hydrophilic protein portions

• Hydrophobic zones: hydrophobic tails + hydrophobic protein portions

• Membrane proteins can move laterally within the layer (fluidity), but rarely flip-flop between layers.

• Proteins are interspersed throughout the lipid bilayer (mosaic aspect).

Functions of Membrane Proteins

• Transport proteins – move substances across the membrane

• Channel proteins – form pores for specific molecules

• Carrier proteins – selectively transport substances

• Receptors – receive signals from outside the cell

• Cell-to-cell recognition – identify cells to each other

Some surface proteins are glycoproteins (proteins with attached carbohydrates), important for cell recognition.

Transport Across Membranes

General Principles

• Movement of molecules across membranes requires:

  1. Passage through a fluid (liquid or gas)

  2. A concentration gradient (difference in molecule concentration across space)

• Movement has a specific net direction, usually from high to low concentration.

Passive Transport

Passive transport is the movement of substances down a concentration gradient, without energy input from the cell.

• Diffusion: Net movement of a substance from higher to lower concentration until equilibrium is reached.

• Occurs over short distances and is a major way substances cross membranes.

• Depends on membrane permeability.

Selective Permeability

Selective permeability is the property of membranes that allows some substances to cross more easily than others.

Facilitated Diffusion

Some substances cross membranes via facilitated diffusion, which uses specific proteins:

• Carrier proteins or channel proteins assist in the movement of selected solutes.

• Types of facilitated diffusion:

  • Bind-and-release (carrier proteins)

  • Selective channels (pores sized for specific solutes)

  • Gated channels (channels that open/close like a door)

Osmosis

Osmosis is the passive transport (diffusion) of water across a selectively permeable membrane.

• Water moves from areas of high water concentration (low solute) to low water concentration (high solute).

• Osmotic pressure measures the tendency of a solution to take up water when separated from pure water by a membrane.

Water Balancing Scenarios

• Isotonic environment: Equal solute concentration inside and outside the cell; no net water movement.

• Hypertonic environment: Higher solute concentration outside; water moves out, cell shrivels.

• Hypotonic environment: Lower solute concentration outside; water moves in, cell swells and may burst.

Cells with walls (e.g., plant cells) are more tolerant to water movement but still undergo changes:

• Isotonic: No net water movement; cell may become limp.

• Hypertonic: Water leaves; cell shrivels.

• Hypotonic: Water enters; cell becomes turgid (firm).

Energy-Requiring Transport Across Membranes

Some transport processes require energy to move substances against their concentration gradient (from low to high concentration).

• Active transport: Uses energy (usually from ATP) to move solutes against a concentration gradient via transport proteins.

• Maintains ion gradients (e.g., Ca2+ ions).

Endocytosis and Exocytosis

• Endocytosis: Cellular uptake of large molecules or particles by engulfing them in vesicles formed from the plasma membrane.

  • Phagocytosis (cell eating): Uptake of solid particles, often involving pseudopodia.

  • Pinocytosis (cell drinking): Uptake of fluids and dissolved substances.

  • Receptor-mediated endocytosis: Specific uptake of macromolecules via binding to cell surface receptors and vesicle formation from coated pits.

• Exocytosis: Secretion of large molecules out of the cell by fusion of vesicles with the plasma membrane.

Energy Flow in the Life of a Cell

Matter and Energy

• Matter: Anything that takes up space and has mass.

• Energy: The capacity to do work.

• Two major categories of energy:

  • Potential energy: Stored energy due to position or structure (e.g., energy in chemical bonds).

  • Kinetic energy: Energy of motion.

Laws of Thermodynamics

• First Law of Thermodynamics: Energy can be transferred and transformed, but cannot be created or destroyed (energy of the universe is constant).

• Second Law of Thermodynamics: Every energy transfer increases the disorder (entropy) of the universe.

• Entropy: Measure of disorder or randomness.

• Closed systems do not exchange energy with surroundings; open systems (like cells) do.

Energy Flow in Chemical Reactions

• All chemical reactions involve reactants (starting substances) and products (ending substances).

• Thermodynamics determines if a reaction will occur and how much energy is consumed or released.

• Reactions are classified as:

  • Exergonic reactions: Release energy (energetically downhill, spontaneous)

  • Endergonic reactions: Require energy input (energetically uphill, non-spontaneous)

Activation Energy

Activation energy is the minimum energy required to start a chemical reaction.

Chemical Equilibrium

Chemical equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction. Concentrations of reactants and products remain constant at equilibrium.

Coupled Reactions

Energy-releasing reactions can be coupled to energy-requiring reactions, allowing cells to drive necessary processes (e.g., using energy from sunlight to power photosynthesis in plants).