Cell Structure and Function: Key Components and Processes

Three main components of a cell

Plasma membrane, cytoplasm, nucleus

Function of the plasma membrane

Defines cell boundary, separates inside from outside, selectively permeable

Model that describes the plasma membrane

Fluid mosaic model (lipid bilayer + proteins)

Integral proteins

Embedded in the lipid bilayer; some have attached carbohydrates forming glycocalyx

Peripheral proteins

Loosely attached to membrane surface; provide support from cytoplasmic side

Functions of the plasma membrane

Barrier, receptors, regulates entry/exit of substances

Simple diffusion

Molecules move down their concentration gradient

Osmosis

Diffusion of water across a membrane

Facilitated diffusion

Molecules move down gradient through an integral protein

Active transport

Uses ATP to move molecules against their gradient

Endocytosis

Process where substances enter a cell via vesicles

Phagocytosis

"Cell eating" - engulfing large particles

Pinocytosis

"Cell drinking" - uptake of fluid/small molecules

Receptor-mediated endocytosis

Specific molecules bind to membrane receptors, form coated vesicle (e.g., insulin, LDL)

Exocytosis

Process that moves substances out of a cell using vesicles

Three parts of the cytoplasm

Cytosol, organelles, inclusions

Cytosol

Jellylike fluid containing water, ions, enzymes

Ribosomes made of

Proteins + ribosomal RNA (rRNA)

Function of ribosomes

Site of protein synthesis (translation)

Difference between free and bound ribosomes

Free ribosomes → proteins for use in cytosol; Bound ribosomes → proteins for export or membranes

Endoplasmic reticulum

Network of membranes in cytoplasm; smooth or rough ER

Function of rough ER

Protein synthesis and transport (studded with ribosomes)

Function of smooth ER

Lipid metabolism, detoxification, calcium storage (no ribosomes)

Golgi apparatus

Stack of cisterns; sorts, modifies, and ships proteins ("packaging/shipping center")

Lysosomes

Membrane sacs with digestive enzymes; break down unwanted material

Mitochondria

Double-membrane organelles; produce ATP ("power plant" of the cell)

Cells with more mitochondria

Cells with high energy needs (e.g., muscle cells)

Special about mitochondria DNA

Contains maternal DNA, thought to originate from bacteria

Peroxisomes

Organelles with oxidase/catalase enzymes; neutralize free radicals, break down toxins and fatty acids

Cytoskeleton

Network of protein rods supporting the cell shape and movement

Centrosome

Region near nucleus with centrioles that organize microtubules

Centrioles

Cylindrical bodies of microtubules; form cilia, flagella, and mitotic spindle

Cytoplasmic inclusions

Temporary structures like pigments, lipid droplets, glycogen

Nucleus

Control center of the cell; holds DNA and directs protein synthesis

Nuclear envelope

Double membrane with pores that allow molecule transport

Nucleolus

Site of ribosome subunit assembly; contains chromosome parts

Chromatin

DNA + proteins forming nucleosomes

Extended chromatin

Active DNA regions used in transcription

Condensed chromatin

Inactive, tightly coiled DNA (storage form)

Chromosomes

Highly organized chromatin; humans have 46 chromosomes

G1 phase

Cell growth, protein synthesis, centrioles start duplicating

S phase

DNA replication (chromatin extended)

G2 phase

Centrioles finish copying, enzymes for division made

M phase

Mitosis (nuclear division) + cytokinesis (cell splits)

Stages of mitosis

Prophase, metaphase, anaphase, telophase

Prophase

Chromatin condenses, spindle forms, nuclear envelope fragments

Metaphase

Chromosomes align at cell equator

Anaphase

Sister chromatids separate and move to poles

Telophase

Chromosomes uncoil, nuclear envelope reforms

Cytokinesis

Cytoplasm divides, forming two daughter cells

Specialized cell functions

Determined by shape and organelle arrangement

Fibroblasts, epithelial cells, erythrocytes

Cells that connect body parts

Skeletal and smooth muscle cells

Cells that move body parts

Fat cells

Cells that store nutrients

Macrophages

Cells that fight disease

Neurons

Cells that gather information

Sperm cells

Cells that are for reproduction

Free radical theory of aging

Cellular damage from by-products of metabolism (free radicals)

Mitochondrial theory of aging

Aging due to reduced energy output from mitochondria

Genetic theory of aging

Aging is programmed in genes; telomeres shorten with each division

Telomerase

Enzyme that prevents telomere degradation