Cell Structure, Function, and Membrane Components: Human Cell Chapter 4

Chapter 4: Cell Lecture Outline

Chapter 4 Lecture Learning Outcomes

What should you be able to describe about human cells after studying Chapter 4?

• Shapes

• Structures

• Components

• Functional features

• How cellular differences reflect specific functions

What should you know about membrane proteins?

• Six major roles/functions

What should you know about membrane transport?

• Passive transport mechanisms

• Active transport mechanisms

What should you know about resting membrane potential (RMP)?

• Definition

• Conditions required to establish it

• Conditions required to maintain it

What should you know about membrane junctions?

• Structure and function of:

• Tight junctions

• Desmosomes

• Gap junctions

What should you know about cell aging and death?

• Actions and processes associated with cell aging and death

4.1a How Cells Are Studied

What is cytology?

• Study of cells

What is required to study cells?

• Microscopes

What units are cells typically measured in?

• Micrometers (μm)

• 1 μm = 1/10,000 cm

What is microscopy?

• Use of a microscope to view small-scale structures

Why are staining techniques used in microscopy?

• Provide contrast

• Make structures easier to see

4.1b Cell Size and Shape

Do cells vary in size and shape?

• Yes

• Great variation exists

What is the size of an erythrocyte (red blood cell)?

• 7-8 μm

What is the size of a human oocyte?

• 120 μm

Are most human cells visible without a microscope?

• No

• Most are microscopic

What cell shapes occur in the human body?

• Spherical

• Cubelike

• Columnlike

• Cylindrical

• Disc-shaped

• Irregular

4.1c Common Features and General Functions

What is the plasma membrane?

• Outer barrier of cell

• Separates internal contents from external environment

What are modifications of the plasma membrane?

• Cilia

• Flagellum

• Microvilli

What is the nucleus?

• Largest structure in cell

• Enclosed by nuclear envelope

• Contains DNA

• Contains nucleolus

• Contains nucleoplasm

What is nucleoplasm?

• Fluid inside nucleus

What is the cytoplasm?

• Cellular contents between plasma membrane and nucleus

• Includes:

• Cytosol

• Organelles

• Inclusions

What is cytosol?

• Intracellular fluid

• Viscous fluid of cytoplasm

• High water content

• Contains dissolved macromolecules

• Contains ions

What are organelles?

• "Little organs"

• Complex organized structures within cells

• Specialized shapes

• Specialized functions

What are the two categories of organelles?

• Membrane-bound organelles

• Non-membrane-bound organelles

What are membrane-bound organelles?

• Organelles enclosed by membranes

• Separated from cytosol

Which organelles are membrane-bound?

• Endoplasmic reticulum

• Golgi apparatus

• Lysosomes

• Peroxisomes

• Mitochondria

What are non-membrane-bound organelles?

• Not enclosed by membranes

• Primarily composed of proteins

Which organelles are non-membrane-bound?

• Ribosomes

• Cytoskeleton

• Centrosome

• Proteasomes

What are inclusions?

• Not organelles

• Temporary storage materials within cytosol

• Continuously added and removed

Examples of inclusions?

• Pigments

• Glycogen

• Triglycerides

What are the major functions of cells?

• Maintain integrity

• Maintain shape

• Obtain nutrients

• Form chemical building blocks

• Harvest energy

• Dispose of wastes

• Divide to form new cells

Why must cells dispose of wastes?

• Prevent accumulation

• Avoid disruption of cellular activities

Why do some cells divide?

• Produce more cells of same type

• Tissue growth

• Replace dead cells

• Maintain tissues

4.2a Lipid Components

What are the characteristics of the plasma membrane?

• Fluid mixture

• Equal parts lipid and protein by weight

• Regulates movement into and out of cell

• Effective nonpolar barrier

What are the major lipid components of the plasma membrane?

• Phospholipids

• Cholesterol

• Glycolipids

What is the structure of a phospholipid?

• "Balloon with two tails"

• Polar hydrophilic head

• Two nonpolar hydrophobic tails

How are phospholipids arranged in the membrane?

• Two parallel sheets

• Tail-to-tail arrangement

• Forms phospholipid bilayer

What forms the internal environment of the membrane?

• Hydrophobic phospholipid tails

Where are phospholipid heads located?

• Directed outward toward water

Functions of the phospholipid bilayer?

• Basic membrane framework

• Keeps cytosol inside cell

• Keeps interstitial fluid outside cell

What is cholesterol's role in the plasma membrane?

• Four-ring lipid molecule

• Scattered within bilayer

• Strengthens membrane

• Stabilizes membrane against temperature extremes

What are glycolipids?

• Lipids with attached carbohydrates

Where are glycolipids located?

• Outer phospholipid region only

Functions of glycolipids?

• Help form glycocalyx

• Create sugar coating on cell surface

What is the glycocalyx?

• Sugar coating on cell surface

What percentage of membrane weight is protein?

• About half

How do membrane proteins behave within the membrane?

• Float within fluid bilayer

• Move within fluid bilayer

What performs most membrane functions?

• Membrane proteins

What are the two structural types of membrane proteins?

• Integral proteins

• Peripheral proteins

What are integral proteins?

• Embedded within phospholipid bilayer

• Often extend across entire bilayer

• Many are glycoproteins

What are glycoproteins?

• Proteins with attached carbohydrate groups

What are peripheral proteins?

• Not embedded in bilayer

• Loosely attached to inner or outer membrane surface

4.2b Membrane Proteins

What are the six major functions of membrane proteins?

• Transport proteins

• Cell surface receptors

• Identity markers

• Enzymes

• Anchoring sites

• Cell-adhesion proteins

What are transport proteins?

• Regulate movement across membrane

Examples of transport proteins?

• Channels

• Carrier proteins

• Pumps

• Symporters

• Antiporters

What are cell surface receptors?

• Bind ligands

• Receive signals

What is a ligand?

• Molecule that binds receptor

Example of cell surface receptor function?

• Neurotransmitter binds muscle cell receptor

• Initiates contraction

What are identity markers?

• Indicate cell belongs to body

• Distinguish healthy cells from cells targeted for destruction

What are membrane enzymes?

• Proteins attached to membrane surfaces

• Catalyze chemical reactions

What are anchoring sites?

• Secure cytoskeleton to plasma membrane

What are cell-adhesion proteins?

• Attach cells to other cells

4.3 Membrane Transport

What are the four major plasma membrane functions?

• Physical barrier

• Regulates movement

• Establishes electrochemical gradient

• Cell communication

What is membrane transport?

• Movement of substances into or out of cell

What are the two major categories of membrane transport?

• Passive transport

• Active transport

What are passive transport processes?

• Require no energy

• Move substances down concentration gradient

• From higher concentration to lower concentration

Types of passive transport?

• Diffusion

• Osmosis

4.3a Passive Processes: Diffusion

What is diffusion?

• Net movement of molecules/ions

• From higher concentration to lower concentration

• Down concentration gradient

What causes diffusion?

• Kinetic energy (energy of motion)

How does temperature affect diffusion?

• Increased temperature

→ increased kinetic energy

→ increased diffusion rate

How does concentration gradient affect diffusion?

• Steeper gradient

→ faster diffusion

What is a concentration gradient?

• Difference in concentration between two areas

What is equilibrium in diffusion?

• Molecules evenly distributed

• No net movement

What is simple diffusion?

• Unassisted movement through phospholipid bilayer

Which substances move by simple diffusion?

• O₂

• CO₂

• Some fatty acids

• Ethanol

• Urea

Characteristics of simple diffusion?

• Small nonpolar solutes

• Not regulated by membrane

• Depends on concentration gradient

• Continues while gradient exists

What is facilitated diffusion?

• Passive transport requiring membrane proteins

• Used by small charged or polar solutes

Types of facilitated diffusion?

• Channel-mediated diffusion

• Carrier-mediated diffusion

What is channel-mediated diffusion?

• Movement through water-filled protein channels

What are ion channels specific for?

• One ion type

What are leak channels?

• Continuously open channels

What are gated channels?

• Usually closed

• Open briefly in response to stimulus

• Important in muscles and nerves

What is carrier-mediated diffusion?

• Carrier protein assists transport

• Substance binds carrier

• Carrier changes shape

• Substance released opposite side

What direction does carrier-mediated diffusion move substances?

• Down concentration gradient

What is a uniporter?

• Carrier protein transporting one substance

What determines the maximum rate of facilitated diffusion?

• Number of channels

• Number of carriers

4.3b Passive Processes: Osmosis

What is osmosis?

• Diffusion of water

• Not solutes

• Through selectively permeable membrane

What is a selectively permeable membrane?

• Allows water passage

• Restricts passage of many solutes

What promotes osmosis?

• Differences in water concentration

• Differences in solute concentration

How are water and solute concentrations related?

• Higher solute concentration = lower water concentration