The Cells is part of the Anatomy and Physiology section which provides High Yield information suitable for the MCAT, an entrance exam for medical schools in the United States.
Table Of Contents
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Cell basics
- – about 100 trillion cells in a human
- – size and shape related to the function
- – in general very small, but a range of sizes
- 1. 8 – 140 μm in diameter, but typically 10 – 20 μm (μm=1/1000 mm or 1/25,000 inch)
- – the Generalized Cell and its major parts
- 1. plasma membrane separates inside from outside
- a. intracellular fluid (ICF)
- b. extracellular fluid (ECF)
- 2. nucleus is the control center
- 3. cytoplasm is everything between the nucleus and the plasma membrane
- a. cytosol – semiliquid portion (ICF), which suspends the other parts and is a site of chemical
reaction - b. organelles – specialized structures with specific functions
- c. inclusions – temporary storage structures
- a. cytosol – semiliquid portion (ICF), which suspends the other parts and is a site of chemical
- 1. plasma membrane separates inside from outside
The Plasma Membrane
- – Structure
- 1. phospholipid bilayer is the basic structure
- a. important for fluidity
- b. is a barrier
- 2. cholesterol
- a. fluidity and stability
- 3. proteins
- a. integral proteins span the membrane, may be channels, transporters, receptors
- b. peripheral proteins are on one side only, may be enzymes or anchors for cytoskeleton
- 4. carbohydrates
- a. on outer surface only, includes glycoproteins and glycolipids (collectively called glycocalyx), important for recognition of self, attachments to other cells
- 1. phospholipid bilayer is the basic structure
- – Basic functions
- 1. communication within body and with non-self cells
- 2. defines boundaries and protects
- 3. maintains chemical and electrical gradients
- 4. selective permeability – controls what gets in and out
- – Membrane Transport
- 1. Passive (cell does not use up its own energy)
- a. diffusion – molecules move down their concentration gradient from greater → lesser
concentration, charged molecules move down electrochemical gradients- 1) simple diffusion – moves through bilayer or protein channel
- 2) osmosis – water moves across bilayer
- 3) facilitated diffusion – uses a protein carrier
- b. filtration – water and solutes forced through membrane by hydrostatic pressure
- a. diffusion – molecules move down their concentration gradient from greater → lesser
- 2. Active (requires the cell to use its ATP)
- a. carrier proteins transport substance against its concentration gradient
- b. endocytosis – substance brought into cell
- 1) piece of membrane surrounds substance and pinches off inside cell (vesicle)
- 2) pinocytosis (cell drinking) and phagocytosis (cell eating)
- c. exocytosis – opposite of endocytosis
- 1. Passive (cell does not use up its own energy)
Nucleus
- – Usually round, at cell center
- 1. double membrane
- 2. has large nuclear pores
- 3. contains nucleoli – parts to make ribosomes
- 4. contains DNA
- a. in the form of chromatin when cell not dividing (long thin strands)
- b. in the form of chromosomes when cell dividing (coiled up)
Organelles
- – Endoplasmic reticulum (ER)
- 1. membranous network of channels
- 2. rough ER
- a. has ribosomes (rRNA plus proteins)
- b. protein and lipid synthesis
- 3. smooth ER
- a. continues processing of rough ER products
- b. specialized in some cells for lipid synthesis or detoxifying chemicals
- – Golgi complex
- 1. stacked membranous sacs
- 2. processing, sorting, packaging of ER products
- 3. makes vesicles for transport to destinations inside cell, or for secretion
- – Lysosomes
- 1. sacs contain digestive enzymes
- a. recycles material from cell
- b. breaks down substances brought in by endocytosis
- 1. sacs contain digestive enzymes
- – Peroxisomes
- 1. sacs contain oxidative enzymes – O2 used to detoxify harmful substances
- – Mitochondria
- 1. double-membraned, makes ATP via cellular respiration
- a. inner membrane has folds called cristae
- b. gel inside called matrix
- 1. double-membraned, makes ATP via cellular respiration
- – Vaults
- 1. probably involved in transport between nucleus and cytoplasm
- – Centrosome
- 1. found near nucleus
- 2. consists of centrioles (protein tubules) surrounded by a centrosome matrix (a cloud of protein)
- 3. organizing center for parts of the cytoskeleton
Cytoskeleton
- – protein filaments running through cytosol
- 1. important in movement of cell and within cell
- 2. supports cell and organelles
- – cell projections for movement made of microtubules
- 1. flagella (on sperm) – one long projection, moves the whole cell
- 2. cilia are many small projections that move substances across the surface of the cell
Inclusions
- – No membrane, temporary storage of products like fat or glycogen
Cell Division
- – Cells must divide for growth, replacement of dead cells, reproduction of the organism (making sperm/eggs)
- – in somatic cells (typical body cells), division involves making two “daughter cells” that are identical to the “parent cell”
- – Cell cycle
- 1. the sequence of events from the time a cell begins to divide until the time the daughter cells divide
- 2. cells divide at different rates
- a. some don’t divide at all, like mature neurons and RBCs
- b. rapid dividers include skin cells and the cells lining the digestive tract
- c. once a cell is going to divide the whole cycle may take 18 – 24 hours
- 3. Interphase
- a. most of the cell’s time spent in this phase
- b. lots of metabolic activity, performing the cell’s usual functions
- c. consists of:
- 1) G1 – growth, can last several hours to days or years, toward the end the centrioles begin replication
- 2) S – DNA synthesis (cell must double the amount of DNA so both daughter cells have the correct genetic material)
- 3) G2 – another growth phase
- 4. Mitosis (nuclear division, lasts about 2 hours)
- a. prophase – chromatin coils up into chromosomes, nucleoli and nuclear membrane break apart, mitotic spindle forms from centrioles
- b. metaphase – chromosomes line up in center of mitotic spindle
- c. anaphase – chromosomes pull apart
- d. telophase – chromosomes uncoil, nucleoli and nuclear membrane form, mitotic spindle breaks down
- 5. Cytokinesis
- a. division of the cytoplasm
- b. in late anaphase or early telophase the plasma membrane indents (cleavage furrow) and pinches into two separate cells
