AP Biology 2.1-2.2: Cell Structure and Function

Eukaryotic cell

A cell with a membrane-bound nucleus, membrane-bound organelles, and multiple linear chromosomes. Its organelles compartmentalize cellular processes.

How does compartmentalization benefit a eukaryotic cell?

Membrane-bound organelles create distinct chemical environments, allowing incompatible reactions to occur efficiently and enabling specialized functions.

What structures distinguish most plant cells from animal cells?

Plant cells typically contain a cellulose cell wall, chloroplasts or other plastids, and a large central vacuole. Animal cells instead commonly contain lysosomes and centrosomes with centrioles.

Plasma membrane

A selectively permeable phospholipid bilayer containing proteins and cholesterol that separates the cell from its environment and regulates movement of ions, water, oxygen, organic molecules, and wastes.

What molecular structure forms the basic framework of the plasma membrane?

A phospholipid bilayer. Each phospholipid has two hydrophobic fatty acid chains and a hydrophilic, phosphate-containing head group, producing an amphipathic membrane.

How do microvilli illustrate the principle that form follows function?

Microvilli are fingerlike projections of the plasma membrane that increase surface area, improving absorption in cells such as those lining the small intestine.

Cytoplasm

The region between the plasma membrane and nuclear envelope, consisting of cytosol, organelles, cytoskeletal components, dissolved ions, and organic molecules. Many metabolic reactions occur there.

Nucleus

The organelle that houses the cell's DNA and directs gene expression, including the synthesis of RNA and proteins.

Nuclear envelope

A double phospholipid bilayer surrounding the nucleus. Nuclear pores regulate movement of ions, molecules, and RNA between the nucleoplasm and cytoplasm, and the envelope is continuous with the endoplasmic reticulum.

Chromatin

The DNA-protein material that makes up eukaryotic chromosomes. It is relatively diffuse during growth and maintenance but condenses into visible chromosomes before cell division.

How do eukaryotic chromosomes differ from the typical prokaryotic chromosome?

Eukaryotic chromosomes are generally multiple, linear DNA molecules associated with proteins, whereas prokaryotes typically have a single circular chromosome.

Nucleolus

A dense region within the nucleus where ribosomal RNA combines with proteins to form ribosomal subunits. The subunits exit through nuclear pores.

What is the relationship between the nucleolus and ribosomes?

The nucleolus assembles ribosomal subunits, which are exported to the cytoplasm and later combine during protein synthesis.

Ribosome

A ribonucleoprotein complex composed of large and small subunits that translates mRNA into a specific sequence of amino acids. Ribosomes may be free in the cytoplasm or bound to the rough ER.

What determines whether a ribosome is free in the cytoplasm or attached to the rough ER?

The destination of the protein being synthesized determines ribosome location. Proteins entering the secretory pathway or becoming membrane proteins are synthesized on ER-bound ribosomes, whereas many cytosolic proteins are made by free ribosomes.

Endomembrane system

A coordinated network of membranes and organelles that synthesizes, modifies, sorts, packages, and transports proteins and lipids. It includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane, but not mitochondria or chloroplasts.

Endoplasmic reticulum (ER)

A membrane network continuous with the nuclear envelope. It includes rough ER, which processes proteins and makes membrane phospholipids, and smooth ER, which synthesizes lipids and steroid hormones, detoxifies substances, and stores calcium ions.

Rough endoplasmic reticulum (RER)

The ER region covered with ribosomes on its cytoplasmic surface. It synthesizes proteins destined for secretion, membranes, or certain organelles and begins their folding and chemical modification in the ER lumen.

Smooth endoplasmic reticulum (SER)

The ER region lacking many ribosomes. It synthesizes lipids and steroid hormones, participates in detoxification, and stores calcium ions; in muscle cells it is specialized as the sarcoplasmic reticulum.

Golgi apparatus

A stack of flattened membrane sacs that modifies, sorts, tags, and packages proteins and lipids received from the ER. Vesicles arrive at the cis face and leave from the trans face.

What is the significance of the cis and trans faces of the Golgi apparatus?

The cis face receives transport vesicles from the ER, whereas the trans face packages materials into vesicles for their final destinations, including secretion, lysosomes, or other cellular membranes.

Describe the route of a protein destined for secretion from a eukaryotic cell.

The protein is synthesized into the rough ER lumen, transported in a vesicle to the Golgi cis face, modified and sorted as it moves through the Golgi, and packaged into a vesicle that fuses with the plasma membrane for exocytosis.

How can a membrane protein synthesized in the ER become part of the plasma membrane?

The protein is inserted into the ER membrane, carried in a transport vesicle to the Golgi, modified and sorted, and then delivered in a vesicle that fuses with the plasma membrane. The vesicle membrane becomes continuous with the plasma membrane.

Why would a protein-secreting cell contain abundant rough ER and Golgi apparatus?

The rough ER synthesizes and initially modifies secreted proteins, while the Golgi further modifies, sorts, and packages them. High secretory activity therefore requires extensive versions of both organelles.

How do vesicles and vacuoles differ functionally?

Both are membrane-bound sacs involved in storage and transport. Vesicles commonly fuse with the plasma membrane or other membranes, whereas vacuoles are generally larger storage or digestive compartments.

Lysosome

A membrane-bound, acidic organelle containing hydrolytic enzymes that digest proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles. Its low pH helps maintain enzyme activity while limiting digestion in the cytoplasm.

Why is the acidic interior of a lysosome useful?

Lysosomal enzymes function best at low pH, while many cytoplasmic reactions do not. Compartmentalization therefore allows macromolecule digestion without broadly disrupting cytoplasmic chemistry.

Peroxisome

A single-membrane organelle that performs oxidation reactions, including fatty-acid and amino-acid breakdown and detoxification. Enzymes such as catalase convert harmful $\mathrm{H_2O_2}$ into water and oxygen.

Mitochondrion

A double-membrane organelle that carries out aerobic cellular respiration and produces ATP. It contains its own DNA and ribosomes.

Mitochondrial matrix

The compartment enclosed by the inner mitochondrial membrane. It contains enzymes for several reactions of cellular respiration, along with mitochondrial DNA and ribosomes.

How do mitochondrial cristae support ATP production?

Cristae are folds of the inner mitochondrial membrane that increase membrane surface area for electron-transfer proteins and ATP synthase.

What are the overall reactants and products of aerobic cellular respiration?

Glucose and oxygen are consumed, while carbon dioxide, water, and ATP are produced. In simplified form: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP}$.

What happens to ATP production when oxygen is unavailable?

Aerobic respiration decreases substantially because oxygen is required as the terminal electron acceptor. Cells may rely on fermentation, which produces much less ATP and can generate lactic acid.

Chloroplast

A plant and algal organelle that carries out photosynthesis. It contains chlorophyll, its own circular DNA and ribosomes, and a double membrane.

What are the major compartments of a chloroplast and their roles?

Light-dependent reactions occur in the thylakoid membranes, which are stacked into grana. Carbon fixation and sugar synthesis occur in the stroma, the fluid surrounding the grana.

What is the overall chemical relationship represented by photosynthesis?

Light energy drives the conversion of carbon dioxide and water into glucose and oxygen: $\mathrm{6CO_2 + 6H_2O + light\ energy \rightarrow C_6H_{12}O_6 + 6O_2}$.

Central vacuole

A large plant-cell compartment that stores water and solutes, helps regulate water balance, supports cell expansion, and contributes to structural rigidity through turgor pressure.

Why does a plant wilt when its central vacuole loses water?

Water loss reduces turgor pressure, so the vacuole no longer pushes firmly against the cell wall. The cell and surrounding tissue lose mechanical support.

Cell wall

A rigid external layer that protects a cell, provides structural support, and helps maintain its shape. Plant cell walls are primarily cellulose, a polysaccharide made of linked glucose units.

How does cellulose differ chemically from starch even though both are glucose polymers?

Cellulose contains glucose units joined predominantly by $\beta(1\rightarrow4)$ linkages, producing strong structural chains. The different linkage geometry gives cellulose properties distinct from starch.

Centrosome

A microtubule-organizing center near the nucleus of animal cells. It contains a pair of perpendicular centrioles and helps organize microtubules, particularly during cell division.

Endosymbiosis

The evolutionary model that mitochondria and chloroplasts originated from free-living bacteria engulfed by ancestral host cells and retained in a mutually beneficial relationship.

What evidence supports the endosymbiotic origin of mitochondria and chloroplasts?

Both organelles have double membranes, circular DNA, their own ribosomes, and bacterial-like dimensions. They also reproduce in ways resembling bacterial division.